The present invention relates to a device for detecting the distribution of a specific pressure in the ground-contacting area of a motor-vehicle tire. The device is of the type comprising: a rest surface, actuator means for bringing the tire under examination to act, by its rolling surface, in a thrust relationship against the rest surface, and detection means operatively interposed between the tire and the rest surface to detect and display the distribution of the specific pressure in the ground-contacting area of said rolling surface on the rest surface.
The invention also pertains to a method carried out by said device for detecting the distribution of a specific pressure in the ground-contacting area of a motor-vehicle tire.
It is known that each tire has a rolling surface, usually defined by a tread band engraved according to a specific pattern, by which contact between the tire and the road-bed occurs. More particularly, contact between the ground and rolling surfaces takes places according to a so-called ground-contacting area, at which all of the forces resulting from vertical loads applied to the wheel are discharged to the ground, as well as tangential efforts generated by the effect of the driving torque or braking torque applied to the wheel itself and by effect of thrusts generated while the vehicle is running.
Usually, the load distribution in the ground-contacting area is never homogeneous. In fact even under static conditions, in the ground-contacting area different regions having different specific-pressure values can be identified, which values can vary, the load carried by the tire axis being equal, depending on a variety of factors such as the configuration of the tread pattern and the mechanical and structural features of the individual components in the tire. In any case, the modalities according to which the distribution of the specific pressures takes place in the ground-contacting area affects many of the functional features of the tire to a great degree, such as sensitivity to the aquaplaning phenomenon, roadholding, wear evenness in the tread band, braking stability, etc.
Therefore, it is important to know the distribution of the specific pressures in the ground-contacting area in order to be able to properly intervene, during the planning step, in the construction features of the tire, the selection of the materials used in making the tire, the selection of the tread pattern and all other factors that may affect such a distribution, in order to give the tire the desired functional features.
In this connection, the methods that have been hitherto known for detection of the specific-pressure distribution essentially consist in identifying the different surface portions of the tread that, following deflection undergone by the tire under load, come into contact with the rest surface of the tire: the value of the corresponding specific pressure is evaluated from the deformation undergone by the tread pattern or the color intensity of the foot prints left by the elements of said tread (ribs and/or blocks) on an appropriate contrast element.
For example, according to a well-known method which has been used for a long time even if only utilizable under static test conditions, the tire is brought to act with a predetermined load, on a flat rest surface, upon interposition of a carbon-paper sheet overlying a white paper sheet, on which the morphology of the ground-contacting area is thus reproduced. More particularly, blackening produced by carbon-paper on the white paper sheet at each point of the ground-contacting area will be much more marked as the specific pressure acting on that point is greater.
In this manner it is possible to achieve a visual representation of the distribution of the specific pressures in the ground-contacting area under static test conditions.
According to a different detection method, to be applied to the tire in motion as well, as described in U.S. Pat. No. 4,095,464, the tire under examination is pressed with a predetermined load, both in an immobilized condition and in motion, against a sheet of glass covered with a colored contrast liquid: on the sheet of glass an image of a black color is produced at the surface of the tread elements directly in contact with the glass and an image of another contrasting color at the voids of the tread pattern, that is at the hollows and at the fins or lamellae. The image thus created is photographed from the side of the free surface of the sheet of glass, by a light-sensitive means having a high definition and sensitivity, thereafter the photographed image thus obtained is optoelectrically converted to an electric signal representative of the tread image under load and said electric signal is compared with a similar reference signal, relating to the discharged tire, thereby drawing therefrom indications on the deformation value of the tread and the specific pressure applied thereto.
All the above means lack precision, are not very reliable and are often very expensive.